1. Field of the Invention
The present invention relates to a laminated balun transformer, more particularly to a laminated balun transformer realized by a structure including three λ/4 strip lines, thereby reducing a size thereof.
2. Description of the Related Art
“Balun” stands for BALanced to UNbalanced. A balun transformer is a balanced-unbalanced signal converter for converting a balanced signal in a balanced transmission line into an unbalanced signal in an unbalanced transmission line and vice versa.
Such balun transformer is used for connecting a component such as a mixer and a Surface Acoustic Wave (SAW) filter using balanced signals, which are widely used in wireless signal processing units, and a component such as an amplifier using unbalanced signals.
A balun transformer is usually implemented by combining a plurality of distribution constant transmission lines or concentrated constant transmission lines. Further, the balun transformer can by realized by resonance waveguides when the balun transformer is applied in the antenna field.
U.S. Pat. No. 6,285,273 issued to Takehiko Morikawa and titled “Laminated Balun Transformer” discloses a conventional laminated balun transformer comprising a plurality of strip lines vertically stacked.
With reference to FIG. 7 and FIG. 8, a conventional laminated balun transformer 101 comprises four dielectric sheets 102c, 102d, 102f, 102g on which four strip lines 104, 105, 108, 109 are provided, respectively, three dielectric sheets 102a, 102e, 102h on which ground electrodes 112, 113, 114 are provided, respectively, and a dielectric sheet 102b on which a lead electrode 103 is provided.
Of a first, second, third and fourth strip lines 105, 106, 108, 109, the first and second strip lines 105, 106, and the third and fourth strip lines 107, 108 are electro-magnetically coupled together to form two pairs of electro-magnetically coupled strip lines. The second and third strip lines 106, 108 are electrically connected to a ground electrode 113 at respective ends 120b, 108b thereof through a via hole.
All the ground electrodes 112, 113, 114 are connected to all the external electrodes 125, 129, 132 in common. The first strip line 104 is connected to an external electrode 131 through the lead electrode 103 at an end thereof, and further connected to an external electrode 128 at the other end thereof. The strip lines 105, 108 are connected to the ground electrode 113 at an end thereof and connected to external electrodes 127, 126, respectively at the other ends thereof. The fourth strip line 109 is connected to the external electrode 128 at an end thereof, and is opened at the other end thereof. That is, the first strip line 104 and the fourth strip line 109 are electrically connected to each other through the external electrode 128.
The external electrodes 125, 129 and 132 serve as ground electrodes, the external electrodes 126, 127 serve as output terminals for balanced signals, and the external electrode 31 serves as an input terminal for an unbalanced signal.
FIG. 9 illustrates an electric equivalent circuit diagram of the laminated balun transformer shown in FIGS. 7 and 8.
The conventional laminated balun transformer with reference to FIGS. 7 to 9 are realized by using four λ/4 strip lines and two couplers, so that a large number of external electrodes are needed, and a configuration and structure of the laminated balun transformer are complicated.